U v **!
XXII. On the Discovery of Palladium ; with Observations on
other Substances found with Platina. By William Hyde
Wollastoii ' iff. IX Sec. R, S>
Read July 4, 1805.
JlIaving some time since purified a large quantity of platina
by precipitation, I have had an opportunity of observing various
circumstances in the solution of this singular mineral, that have
not been noticed by others, and which, I think, cannot fail to
be interesting to this Society.
As I have already given an account of one product obtained
from that ore,which I considered as a new metallic substance, and
denominated Rhodium, I shall on the present occasion confine
myself principally to those processes by which I originally
detected, and subsequently obtained another metal, to which I
gave the name of Palladium, from the planet that had been
discovered nearly at the same time by Dr. Olbers.
In the course of my inquiries I have also examined the many
impurities that are usually mixed with the grains of platina, but
I shall not think it necessary to describe minutely substances
which have already been fully examined by others,
§ I. Ore of Iridium.
I must however notice one ore, that I find accompanies the
ore of platina, but has passed unobserved from its great re-
semblance to the grains of platina, and on that account is
Dr, Woixaiston on the Discovery of Palladium. 517
scarcely to be distinguished or separated from them, excepting
by solution of the platina ; for the grains of which I speak are
wholly insoluble in nitro-muriatie acid. When tried by the file*
they are harder than the grains of platina ; under the hammer
they are not in the least degree malleable ; and in the fracture
they appear to consist of kminse possessing a peculiar lustre ;
so that although the greater number of them cannot, as I have
before observed, be distinguished from the grains of platina*
the laminated structure sometimes occasions an external form
»y which they may be detected/ With a view to be absolutely
certain that there e^ist grains in a natural state, which have
not been detached by solution from the substance of the grains
of platina, I have Separated from the mixed ore as many as
enabled me to ascertain their general composition.
Their most remarkable quality is their great specific gravity,
which I have fotmd to be as much as 19,5, while that of the
crude grains of platina has not, in any experiment that I have
made, exceeded* 7,7. From this circumstance it might naturally
be cortjectured that they contain a greater quantity of platina
than the grains in general ; by analysis, however, they do not
appear to me to contain the smallest quantity of that metal, but
to be an ore consisting entirely of the metals that were found
by Mr. Tennant in the black powder which is extricated by
solution from the grains of platina, ahd which he has called
Iridium and Osmium, But, since the specific gravity of these
grains so much exceeds that of the powder, which by my ex-
periments has appeared to be, at the utmost, 14,2, I have
thought it might deserve inquiry whether their chemical com-
position is in any respect different. For this purpose I have
selected a portion of them, and have requested Mr. Tennant
318 Dr. Woll Aston on the Discovery of Palladium.
to undertake a comparative examination, from whose well
known skill in chemical inquiries, as well as peculiar knowledge
of the subject, we have every reason to expect a complete ana-
lysis of this ore.
§ .II. Hyacinths.
Among those bodies which may be separated from the ore
of platina, in consequence of their less specific gravity, by a
current of water or of air, there may be discerned a small
proportion of red crystals so minute, that 100 of the largest I
could collect weighed scarcely -X. of a grain. The quantity
which I possess is consequently too small for chemical analysis ;
but their physical properties are such as correspond in every
respect with those of the hyacinth. I was first led to compare
them with that stone by their specific gravity, which I con-
jectured to be considerable from their accompanying other
substances, that appear to have been collected together solely
by reason of their superior weight.
Like the hyacinth, these crystals lose their colour imme-
diately and entirely when heated ; they also agree with it in
their hardness, which is barely sufficient to scratch quartz, but
is decidedly inferior to that of the topaz.
The principal varieties of their form may be very well un-
derstood by description.
ist. In its most simple state the crystal may be considered
as a rectangular prism terminated by a quadrilateral obtuse
pyramid, the sides of which sometimes arise direct from the
sides of the prism ; but,
sdly. The position of the pyramid is generally such that its
sides arise from the angles of the prism. In this case the sides
of the prism are hexagons.
Br. Wollaston on the; Discovery of Palladium, 319
Sdly. It is more usual for the prism to have eight sides by
truncation of each of its angles , and at each extremity eight
additional surfaces occupying the place of the eight linear
angles between the prism and terminating pyramid of the 2d
variety. The complete crystal has then thirty-two sides.
4,thly. The eight surfaces last mentioned, as interposed
between the prism and pyramid, are sometimes elongated into
a complete acute pyramid having eight sides arising from the
angles of an octahedral prism.
The 3d form above described, corresponds so entirely with
that given by the Abbe Hauy * as one of the forms of the
hyacinth or jargon, that I have little reason to regret my inabi-
lity to obtain chemical evidence of the composition of these
crystals.
Those, and other impurities, I usually separated, as far as
was practicable, by mechanical means, previously to forming
the solution of platina, which has been the, principal object of
my attention.
§ III. Precipitation of Platina.
j When a considerable quantity of the ore had been dissolved,
and I had obtained, in the form of a yellow triple salt, as much
of the piatina as could be precipitated by sal ammoniac, clean
bars of iron were next immersed in the solution for the purpose
of precipitating the remainder of the platina.
For distinction it will be convenient to call this, which in fact
consists of various metals, the first metallic precipitate.
The treatment of this precipitate differed in no respect from
that of the original ore. It was dissolved as before, and a portion
* Traiti de Mineralogies PI. XLL fig. iy*~~jQnrn* des Mines, No. 26* fig. 9,
$sq Dr. Wollaston on the Discovery of Palladium.
of platina precipitated by sal ammoniac; but it was observable
that the precipitate now obtained was not of so pale a yellow as
the preceding. Nevertheless the impurity was in so small quan-
tity, that the platina reduced from it by heat did not differ
discernibly from that obtained from the purest yellow preci-
At this time I found it advantageous to neutralize the solution
with soda, and to employ a solution of green sulphate of iron
for the precipitation of the gold, of which, I believe, a portion
may always be obtained from the mixed ore ; but I have ob-
served in experiments upon any quantities of mere grains of
crude platina carefully selected, that the smallest portion of
gold could not be detected as a constituent part of the ore
Bars of iron were subsequently employed as before for re-
covering the platina that remained dissolved, together with
those substances which I have since found to accompany it.
The precipitate thus obtained, which I distinguish by the
name of the second metallic precipitate, was to appearance of
a blacker colour than the former, and was a finer powder.
As I was not at first prepared to expect any new bodies, I
proceeded to treat the second precipitate, as the former, by so-
lution and precipitation. But I soon observed appearances
which I could not explain by supposition of the presence of any
known bodies, and was led to form conjectures of future disco-
veries, which subsequent inquiry has fully confirmed.
When I attempted to dissolve this second metallic precipitate
in nitro-muriatic acid, I was surprised to find that a part of it
resisted the action of that solvent, notwithstanding any varia-
tions in the relative proportions or strength of the acids
Dr. WollastOm m ike Discovery of Palladium. 321
employed to form the compound, and although the whole of this
powder had certainly been twice completely dissolved.
The solution, formed in this case was of a peculiarly dark
colour, and when I endeavoured to precipitate the platina from
it by sal ammoniac^ the precipitate obtained was small in quan-
tity, and, instead of being yellow, was of a deep red colour,
arising from an impurity which I did not at that time understand,
but which we since know, from the experiments of Mr. Des-
cotils, is occasioned by the metal now called iridium.
The solution, instead of being rendered pale by the precipita-
tion of the platina, retained its dark colour in consequence of
the other metals that remained in solution ; but, as I had not
then learned the means of separating them from each other,
and as the quantity of fluid which accumulated occasioned me
some inconvenience, I decqnjposied it by iron, as in the former
instances, and formed a thirdr metallic precipitate, which could
more commpdiously be reserved for subsequent examination.
In this last step I committed an error which afterwards occa-
sioned me considerable difficulty, for I found that a great part
of this precipitate consisting of rhodium was unexpectedly ren-
dered insoluble by this treatment, and resembled the residuum
of the second metallic precipitate abovementioned.
; As I have already communicated to this society, in my Paper
upon rhodium, the process by which I subsequently avoided
this difficulty, I shall at present return to a previous stage of
my progress, and relate the means by which I first obtained
palladium in my attempts to analyze the second metallic pre-
cipitate.
MDCCCV. T t
g%B Dr. Wol&aston m the- -Discovery: of JP<$tadi}tm*
§ IV. Separation of P@iladmm>
There was no difficulty in ascertaining the presence of lead
as one of the ingredients of this precipitate, hy Means of mu-
riatic acid, which dissolved lead and iron and a small quantity
of copper. It was equally easy to obtain a larger portion of
copper by dilute nitrous acid, with which it formed as usual a
blue solution. But when I endeavoured to extract the whole
of the copper by a stronger acid, it was evident, from the dark
brown colour of the solution, that some other metallic ingre-
dient had also been dissolved. I at first ascribed this colour
to iron; but, when I considered that this substance bad been
more slowly acted upon than copper, I relinquished that hy-
pothesis, and endeavouring to 1 precipitate a portion of it by a
clean plate of copper, I obtained a black powder adhering to a
surface of platina on which I had placed the solution. As
this precipitate was soluble in nitric acid; it evidently consisted
neither of gold nor platinaf; as tfe solution in that acid wis of
a red colour, the metal could not be either silver or Mercury ;
and as the precipitation of it by copper excluded :the supposi-
tion of all other known metals, I had reason to sitispieet the
presence of some new body, but was not fully satisfied
of its existence until I attempted the precipitation of it by
mercury.
For this purpose I agitated a small quantity <# mercury in
the nitrous solution previously warmetf, and observed ^he mer-
cury to acquire the consistence of an amalgam. After this
amalgam had been exposed to a red heat, there remained a
white metal, which could not be fused before the blow pipe. It
gave a red solution as before in nitrous acid ; it was not
Dr. Woll aston on the Discovery of Palladium. gag
precipitated by sal ammoniac, or by nitre ; but by prussiate of
potash it gave a yellow or orange precipitate; and in the order
of its affinities it was precipitated by mercury but not by silver*
These are the properties by which I originally distinguished
palladium ; and by the assistance of these properties I obtained
a sufficient quantity for investigating its nature more fully.
There were, however, various reasons which induced me to
relinquish the original process of solution in nitrous acid and
precipitation by mercury ; for although I found the metal thus
obtained to be nearly pure, the necessity of agitating the solu-
tion with the mercury was very tedious, and the waste was also
considerable; for in the first place it seemed that nitrous acid
would not extract all the palladium from any quantity of the
second metallic precipitate, neither would mercury reduce the
whole of what was so dissolved. I therefore substituted a
process dependent on another of its properties. I had observed
that this m&tal differed from platina in not being precipitated
from nitro-muriatic acid by nitre or by other salts containing
potash ; for although a triple salt is thus formed, this salt is
extremely soluble, while that of platina on the contrary re-
quires a large quantity of water for its solution. On that
account a compound menstruum consisting of nitrate of potash
dissolved in muriatic acid is unlit for the solution of platina,
but dissolves palladium nearly as well as common nitro-mu-
riatic acid in which there is no potash present*
In five ounces of muriatic acid diluted with an equal quantity
of water, I dissolved one ounce of nitre, and formed a solvent
* I have found that gold may also be dissolved with equal facility by the same
solvent, and nearly in the same proportion. Ten grains of nitre added t > a proper
quantity of muriatic acid are sufficient for sixteen grains of either gold or palladium*
T4-
324 Dr. Woll aston on the Discovery of Palladium*
for palladium that possesses little power of acting on platina,
so that by digesting any quantity of the second metallic preci-
pitate till there appeared to be no farther action, I procured
a solution from which by due evaporation were formed crystals
of a triple salt, consisting of palladium combined with muriatic
acid and potash. These are the crystals which I have on a
former occasion * mentioned as exhibiting a very singular
contrast of colours, being bright green when seen transversely,
but red in the direction of their axis ; the : general aspect,
however, of large crystals is dark brown.
From the salt thus formed and purified by a second crys-
tallization, the metal may be precipitated nearly pure by iron
or by zinc, or it may be rendered so by subsequent digestion
in muriatic acid.
§ V, Reasons for thinking Palladium a simple Metal.
From the consideration of this salt alone I thought it highly
probable that the substance combined in it with muriate of
potash was a simple metal, for I know of no instance in chemistry
of a distinctly crystallized salt containing more than two bases
combined with one acid. I nevertheless endeavoured by a suit-
able course of experiments to obviate all probable objections.
After examining by what acids it might be dissolved and by
what reagents it might be precipitated, I combined it with
various metals, with platina, with gold, with silver, with
copper, and with lead ; and when I had recovered it from its
alloys so formed, I ascertained that, after every mode of trial
it still retained its characteristic properties, being soluble in
nitrous acid, and precipitable from thence by mercury, by green
* Phil, Traais. 1804, p, 428*
Dn Wollaston on the Discovery of Palladium. 325
sulphate of iron, by muriate of tin, by prussiate of potash, by
each of the pure alkalis, and by hydrosulphurets.
The precipitate obtained in each case was also found to be
reducible by mere heat to a white metal, that, except in very
small quantities, could not be fused alone by the blowpipe, but
could very readily be fused with sulphur, with arsenic, or
with phosphorus, and in all other respects resembled the
original metal.
The only hypothesis, on which I thought it possible that I
could be deceived, arose from the recollection of the error ,
which subsisted for a few years, respecting the compound
formerly called siderite. It was possible that some metallic or
other fixed acid might unite too intimately with either a known
or an unknown metal to be separated by the more common
simple affinities. I consequently made such attempts as ap-
peared best calculated to disunite a compound so constituted.
Having boiled the oxide with pure alkalis, and found it to be
unaltered, I thought the affinities of lime or lead might be
more likely to detect the presence of the phosphoric or of any
known metallic acid ; and accordingly I made various attempts
by muriate and nitrate of lime, as well as by nitrate of lead, to
effect a decomposition of the supposed compound. In the ex-
periment on which I placed the greatest reliance, I poured
liquid muriate of lime into a solution of palladium in nitro-
muriatic acid, and evaporated the mixture to dryness, intending
thereby to expel any excess of acid that might have been left
in the solution, and to render either phosphate of lime, or any
compound of lipie with a metallic acid, insoluble in water. The
residuum however was very readily dissolved by water, and
g%6 Dr. Won aston on the Discovery of Palladium.
consisted merely of muriate of lime and muriate of palladium,
without any appearance of decomposition.
When I fcund all my endeavours directed to that end wholly
unsuccessful, I no longer entertained, any doubt of this sub-
stance being a new simple metal, and accordingly published a
concise delineation of its character ; but by not directing the
attention of chemists to the substance from which it had been
extracted, I reserved to myself an opportunity of examining
more at leisure many anomalous phenomena, that had occurred
to me in the analysis of platina, which I was at a loss to explain,
until I had learned to distinguish those peculiarities, that I
afterwards found to arise from the presence of rhodium.
§ VI. Additional Properties of Palladium.
In my former Paper on that subject I also added some ob«*
servations upon the properties and origin of palladium, describing
only such a mode of obtaining it from platina as should avoid
the introduction of any unnecessary ingredient which might
possibly be misinterpreted, and omitted one of the most dis-
tinguishing properties of palladium, by means of which it may
be obtained with the utmost facility by any one who possesses
a sufficient quantity of the ore of platina.
To a solution of crude platina, whether rendered neutral by
evaporation of redundant acid, or saturated by addition of
potash, of soda, or ammonia, by lime or magnesia, by mercury,
by topper, or by iron, and also whether the platina has or has
not been precipitated from the solution by sal ammoniac, it is
merely necessary to add a solution of prussiate of mercury, for
the precipitation of the palladium. Generally for a few seconds,
Dr. Woiaaston on the Discovery of Palladium. g%j
and sometimes far a few minutes , there will be no appearance
pf any precipitate ; but in a short time the whole solution bet
^omes slightly turbid, and a ftoceulent precipitate is gradually
formed, of a pale^ yellowish-white colour. This precipitate
consists wholly of prussiate of palladium, arid when heated
will be found to yield that metal in a pure state, amounting
to about 4 or 5 tenths per cent, upon the quantity of ore
dissolved.
The prussiate of mercury is peculiarly adapted to the pre-
cipitation of palladium, exclusive of all other metals, on account
of the great affinity of mercury for the prussic acid, which in
this case prevents the precipitation of iron or copper ; but the
proportion of mercury does not by any means influence the
quantity of palladium, for I have in vain endeavoured, In
the above experiment on crude platina, to obtain a larger
quantity of palladium than Ihave stated by using more of the
prussiate of mercury, or to procure any precipitate by the mm®
means from a solution of pure platina.
The decomposition of muriate of palladium by prussiate of
mercury is not effected solely by the superior affinity of mer-
cury for the muriatic acid, but is assisted also by the greater
affinity of prussic acid for palladium; for I have found that
prussiate of palladium may be formed by boiling a precipitated
oxide of palladium in a solution of prussiate of mercury.
The prussiate of mercury is consequently a test by which
the presence of palladium may be detected in any of its solu-
tions ; but it may be worth observing, that the precipitate
obtained has not in all cases the same properties. In general,
this compound is affected by heat similarly to other prussiates,
but when the palladium has been dissolved m nitrous acid and
328 Dr. Woll aston on the Discovery of Palladium.
precipitated from a neutral solution by prussiate of mercury,
the precipitate thus formed has the property of detonating
when heated. The noise is similar to that occasioned by firing
an equal quantity of gunpowder, and accordingly the explo-
sion is attended with no marks of violence unless occasioned
by close confinement. The heat requisite for this purpose is
barely sufficient to melt bismuth, consequently is about 500 of
Fahrenheit. The light produced is proportionally feeble, and
can only be seen in the absence of all other light.
In endeavouring to dissolve a piece of palladium in strong
colourless nitric acid for the purpose of forming the detonating
prussiate, I found that, although the acid shortly acquired a red
colour surrounding the metal, the action of the acid was ex-
tremely slow, and I was surprised to observe a fact that appears
to me wholly singular : the metal was taken up without any
extrication of nitrous gas ; and this seemed to be the cause of
the slow solution of this metal, as there was not that circula-
tion of the fluid, which takes place in the solution of other
metals until the acid is nearly saturated.
As the want of production of gas appeared to retard the solu-
tion of palladium, I tried the effect of impregnating a quantity
of the same acid previously with nitrous gas, and observed its
action to be very considerably augmented, although the expe-
riment was necessarily tried in the cold, because the gas would
Iiave been expelled by the application of heat.
Beside those properties which are peculiar to palladium there
are others, not less remarkable, which it possesses in common
with platina. I have on a former occasion mentioned that these
inetals resemble each other in destroying the colour of a large
quantity of gold. Their resemblance, however, in other
Dr. Wollaston on the Discovery of Palladium. 329
properties is not less remarkable, more especially in the little
power they possess of conducting heat, and in the small degree
of expansion to which they are liable when heated.
For the purpose of making a comparison of the conducting
powers of different metals, I endeavoured to employ them in
such a manner, that the same weight of each metal might ex-
pose the same extent of surface. With that view I selected
pieces of silver, of copper, of palladium, and platina, which had
been laminated so thin as to weigh each 1 o grains to the square
inch. Of these I cut slips y^ of an inch in breadth, and four
inches long ; and having covered their surfaces with wax, I
heated one extremity so as to be visibly red, and, observing
the distance to which the wax was melted, I found that upon
the silver it had melted as far as 3^ inches : upon the copper «|
inches : but upon the palladium and upon the platina only 1
inch each : a difference sufficient to establish the peculiarity of
these metals, although the conducting power cannot be said to
be simply in proportion to those distances.
In order to form some estimate of the comparative rate of
expansion of these metals, I rivetted together two thin plates of
platina and of palladium; and observing that the compound plate,
when heated, became concave on the side of the platina, I ascer-
tained that the expansion of palladium is in some degree the
greater of the two. By a similar mode of comparison I found
that palladium expands considerably less than steel by heat ; so
that if the expansion of platina between the temperatures of
freezing and boiling water be estimated at 9 parts in 10,000,
while that of steel is known to be about 12, the expansion of
palladium will probably not be much more or less than 10, or
one part in 1000 by the same difference of temperature.
MDCCCV. U U
330 Dr. Wollaston on the Discovery of Palladium.
It must, however, be acknowledged that the method I have
pursued is by no means sufficient for determining the precise
quantity of expansion of any substance ; but I have not been
induced to bestow much time on such an inquiry, since the
extreme scarcity of palladium precludes all chance of any prac-
tical utility to be derived from a more accurate investigation.
